512 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



[Feb. 2, 



called by the natives *' the burnt field." Its surface is covered, like 

 the "Malpais" of Mexico, with a thick bed of ashes, and from this 

 rise numerous small knolls or hillocks some 10 or 12 feet high, and 

 about the same in diameter at the base, from each of which issued, 

 at the time of their visit, ^' fumaroles," or streams of hot vapour 

 having an odour of sulphuretted hydrogen. Here, as at JoruUo, it is 

 evident that abundant showers of ashes gave to the superficial 

 asperities of the great lava-bed (which discharged vapours as usual, 

 so long as the interior retained its heat) the same conical or dome- 

 shaped figures that we see given by a deep fall of snow to accidental 

 protuberances on the surface of any rough field. Indeed, I observed a 

 precisely similar phenomenon produced by the eruption of Vesuvius 

 of 1822, as I remarked in a paper read before this Society in 1827. 



The upheaval or " blister" theory having been, however, thus 

 originated by M. de Humboldt, was adopted, and further developed 

 by M. de Buch in his work on the Canary Isles. 



This author says, of the Peak of Teneriife itself (fig. 4), notwith- 

 standing that he describes its only visible part (the surface) to be 

 composed of ejected pumice and streams of glassy lava, which are 

 admitted to have repeatedly flowed down the steep sides of the cone 



Fig. 4. — PeaJc of Teneriffe, as seen from the margin of the Cirque. 



oei'iii''^ 



from its summit * — thereby showing the cone to be of a purely erup- 

 tive origin, so far as it can be observed : — 



" It (^. e. the entire cone of the Pic de Teyde) has been produced 

 by the ' upheaval ' of a mass urged upwards by the force beneath, 

 which struggled for a vent, and which while forcing a passage in 

 the middle of the crater of elevation (the surrounding cirque) lifted 

 up the mass above it in the form of a domef." 



This assertion, be it remarked, is made dogmatically, without any 

 attempt to support by argument so strange a proposition, and one so 

 contrary to received notions — nay, even to the facts he himself 

 relates as to the cone being visibly composed of loose pumice and 

 lava-streams. 



In the same manner, in speaking of Yesuvius, in the latter part 

 * Canaries, p. 196. f Ibid. p. 202. 



